1. Field of the Invention
This invention relates to apparatus and a method for determining the amount of substance in a vessel.
2. Description of the Prior Art
In the pharmaceutical and allied industries, the vessels in which liquids are processed must be free from bacteria and other undesired organisms. Therefore, it is necessary for the vessels to be as smooth as possible so that sites where such organisms can grow are minimised.
Consequently, it is very difficult to measure the amount of liquid in a vessel. Float chambers at the side of the vessel or sight glasses built into the vessel cannot be used since these would provide sites where cleaning is difficult and undesired micro-organisms could breed and flourish.
Methods have been devised to overcome these problems of measurement. For example, one method includes the use of a tape measure to determine the distance between the surface of the liquid and the top of the vessel. This distance can then be used to calculate the volume of liquid in the vessel.
This method suffers from the disadvantages that the tape measure is sometimes not held vertical, and it is difficult to determine when the end of the tape measure is just in contact with the surface of the liquid. In addition, these errors will be made worse by any reading errors on the tape.
Often, particularly when the liquid is agitated in processing, foam is formed on the surface. This makes measurement by the method described above extremely difficult but also renders measurement by radar or sonar techniques inaccurate as it is difficult to distinguish between foam on the surface of the liquid and the surface itself.